1. Field of the Invention
The present invention relates to a method of recording a radiation image of a region of interest of an object. The present invention is applicable in a method for composing a radiation image of a long length object by radiation images of parts of the long length object and for accurately locating each of these parts of the long length object.
2. Description of the Related Art
In X-ray radiography an x-ray image of a long length object (also called elongate object), such as the entire spine or the legs of a patient, may have to be obtained.
In Computed Radiography (CR), such a long length image is generated by simultaneously subjecting a number of Imaging Plates (IP), such as photo-stimulable phosphor plates, which are organized in a partially overlapping disposition to an x-ray image of the long length object. Each of the imaging plates carries an image of a part of the long length object. After exposure, the individual imaging plates are read out so as to obtain partial images of the long length object and finally a long length image is created by stitching these partial images. Accurate alignment and measurement can be obtained by superimposing a grid of radiation attenuating material covering the region to be imaged and correcting and aligning the partial images to reconstruct the geometry of said grid.
In recent years, Digital Radiography (DR) has become a valuable alternative for CR. The flat panel detectors (FPD) used in DR are more costly than the IP's for CR, so an alternative to the one-shot long length imaging technique of CR using multiple Imaging Plates is needed. This is achieved by taking plural partial images of a long length object by moving the position of the FPD while tilting the X-ray tube and/or moving the X-ray tube parallel to the FPD and/or collimating.
In one embodiment a method for positioning the x-ray source for the generation of the different partial images generally consists of the following steps:                determining the total length of said long length object to be imaged,        calculating on the basis of said determined length and (a) predefined amount(s) of overlap between the partial images, the number of partial images required to image said long length object,        determining on the basis of the calculated number of partial images, the size and position of said partial images, to cover the long length object,        consecutively exposing parts of the long length object to generate partial images,        detecting partial images by a radiation detector and reading out said partial images and        pasting said partial images to form said image of said long length object.        
In order to be able to paste the partial images so as to make a correct reconstruction of the entire long length image, an object of known geometric dimensions, e.g. a grid consisting of wires of x-ray attenuating material or a grid comprising x-ray attenuating markers at known positions, is positioned together with the long length object in the beam of radiation so that the radiation image of each time another part of the object of known dimensions (e.g. a grid) appears superposed on each of the partial images. By reconstructing the image of the entire grid, the partial images of the long length object can accurately be recomposed to form the complete radiation image of the long length object.
In order to record the partial images, the source of x-rays has to be positioned and occasionally tilted and collimated so that the emitted beam of radiation successively irradiates the different juxtaposed or slightly overlapping parts of the long length image.
This is achieved via parameters (coordinate) settings applied to a controlling device controlling the positioning and/or tilting and/or collimation of the radiation source.
However, adjustment inaccuracies might occur when adjusting the settings of the x-ray source. Specifically in applications in which a long length object is exposed when it is in a horizontal position on a supporting table, it might occur that the relative position of the supporting table and/or of the grid used for reconstructing the long length image from the partial images is unknown or has changed with respect to an expected position. This might be due to the fact that the table and/or grid may have shifted in the horizontal direction and may thus have been displaced relative to the source position.
This might cause an inaccurate setting of the x-ray source.